The present invention relates to electrical power generation. More specifically, the present invention relates to a permanent magnet generator and control.
Constant voltage, variable frequency (CVVF) aircraft electrical distribution systems are being considered by commercial airframe companies. The CVVF systems would be required to deliver 115 volt, three phase electric power at frequencies varying between approximately 350 Hz, and 760 Hz to on-board electronics such as avionics, vapor cycle systems, and electrically driven hydraulic compressors.
A so-called “Two Pole Toothless” permanent magnet generator would be desirable for CVVF systems because of its robust rotor design and a low magnetic spring rate associated with a large air gap. This permanent magnet generator could be driven at prime mover speed (e.g., 24,000 rpm) without an intermediate gear train and constant speed drives. These features would allow the permanent magnet generator to be integrated directly with a prime mover such as an aircraft engine or an auxiliary power unit (APU). Permanent magnet rotors of the generator would form a single rotating unit with compressor and turbine wheels of the prime mover.
Despite their mechanical robustness, permanent magnet generators are not preferred for aircraft generator applications because 1) their output voltage increases essentially proportionally to speed, and 2) they do not survive short circuits applied to their terminals. Because of their low impedance, short circuit currents in excess of the machine current rating can flow under short circuit applications, causing excessive heat build up in the generator stator windings. A short in the stator windings can fuse the stator windings and destroy the generator. If a generator cannot survive the short circuit conditions, it cannot recover upon short circuit removal and it cannot deliver power.
Moreover, the excitation of permanent magnet generators is fixed, since it is provided by the permanent magnet contained within the rotor. Thus, voltage regulation becomes an issue, since the voltage is proportional to the speed of the rotor.
There is a need for a high-speed permanent magnet generator that can gracefully survive short circuit conditions. There is also a need for a control that can regulate the terminal voltage for a permanent magnet generator.